1. Field of the Invention
The present invention generally relates to the operation of instrumentation within a wellbore. More particularly, the invention relates to an apparatus and a method for conveying and operating tools into a wellbore.
2. Description of the Related Art
In the drilling of oil and gas wells, a wellbore is formed using a drill bit that is urged downwardly at a lower end of a drill string. After drilling a predetermined depth, the drill string and the drill bit are removed, and the wellbore is lined with a string of steel pipe called casing. The casing provides support to the wellbore and facilitates the isolation of certain areas of the wellbore adjacent hydrocarbon bearing formations. An annular area is thus defined between the outside of the casing and the earth formation. This annular area is typically filled with cement to permanently set the casing in the wellbore and to facilitate the isolation of production zones and fluids at different depths within the wellbore. Numerous operations occur in the well before or after the casing is secured in the wellbore. Many operations require the insertion of some type of instrumentation or hardware within the wellbore. For instance, logging tools are employed in the wellbore to determine various formation or structural parameters including hydrocarbon saturation and cement bond integrity.
Early oil and gas wells were typically drilled in a vertical or near vertical direction with respect to the surface of the earth. As drilling technology improved and as economic and environmental demands required, an increasing number of wells were drilled at angles which deviated significantly from vertical. In the last several years, drilling horizontally within producing zones became popular as a means of increasing production by increasing the effective wellbore wall surface exposed to the producing formation. It is not uncommon to drill sections of wellbores horizontally (i.e. parallel to the surface of the earth) or even “up-hill” where sections of the wellbore were actually drilled toward the surface of the earth.
The advent of severely deviated wellbores introduced several problems in the performance of some wellbore operations. Conventional logging was especially impacted. Conventional logging utilizes the force of gravity to convey logging instrumentation into a wellbore. Gravity is not a suitable conveyance force in highly deviated, horizontal or up-hill sections of wellbores. Numerous methods have been used, with only limited success, to convey conventional instrumentation or “tools” in highly deviated conditions. These methods include the use of conveyance members such as electric wireline, slickline, coiled tubing, or jointed pipe.
Electric wireline or “wireline” is generally a multi-strand wire or cable for use in oil or gas wells. Wireline typically comprises at least a single conductor cable surrounded by a plurality of braided non-conductive cables. The non-conductive cables provide structural support for the single conductor cable during transport of the wireline into the wellbore. In a logging operation, a logging tool is attached to the wireline and then the tool string is either lowered into the wellbore utilizing the force of gravity or pulled into the wellbore by a tractor device. As discussed above, gravity is not a suitable conveyance force in highly deviated, horizontal, or up-hill sections of wellbores, and tractor devices are expensive and complex.
A slickline is generally a single-strand non-conductive wire with an outer diameter between 5/16″ to ⅜″. Due to the slickline's small diameter (particularly in relation to typical wellbore diameters) and hence minimal columnar buckling resistance, slickline cannot be pushed or urged into the wellbore, but rather slickline must rely on utilizing the force of gravity. For example, in a logging operation, a logging tool is attached to the slickline and then the tool string is lowered into the wellbore utilizing the force of gravity. As discussed above, gravity is not a suitable conveyance force in highly deviated, horizontal, or up-hill sections of wellbores. Both slickline and wireline are “impossible” to push for any appreciable distance in a wellbore. The old adage “like pushing a rope” indicating extremely difficult is applicable to attempts to deploy wireline or slickline by means of axial force applied from the surface (of the Earth). The structural component of wireline is typically braided cable. As such, wireline performs reasonably well under axial tension, but particularly poorly under axial compression. The buckling strength of wireline having a given apparent diameter is greatly diminished because the strands of cable comprising the wireline are capable of relative axial movement. Even slickline, which is typically single strand construction, has a fairly low buckling strength because of its small diameter (and therefore large length to diameter ratio when deployed).
Coiled tubing can be “pushed” into a wellbore more readily than wireline or slickline but still has limitations. Coiled tubing is a long continuous length of spooled or “reeled” thin walled pipe. Coiled tubing units utilize hydraulic injector heads that push the coiled tubing from the surface, allowing it to reach deeper than slickline, but ultimately the coiled tubing stops as well. Coiled tubing is susceptible to a condition known as lockup. As the coiled tubing goes through the injector head, it passes through a straightener; but the tubing retains some residual bending strain corresponding to the radius of the spool. That strain gives the tubing a helical form when deployed in a wellbore. Therefore it winds axially along the wall of the wellbore like a long, stretched spring. Ultimately, when a long enough length of coiled tubing is deployed in the well bore, frictional forces from the wellbore wall rubbing on the coiled tubing cause the tubing to bind and lock up, thereby stopping its progression. Such lock up limits the use of coiled tubing as a conveyance member for logging tools in highly deviated, horizontal, or up-hill sections of wellbores.
Another limitation of coiled tubing is the limited capability of pushing coiled tubing into the wellbore due to known structural characteristics of coiled tubing. Coiled tubing comes in a range of diameters and wall thicknesses. For instance, coiled tubing could have a 1″ diameter with a wall thickness of 0.080″ to approximately a 3.5″ diameter with a wall thickness of 0.203″. The ability of a pipe to withstand buckling under axial end loading is proportional to the pipe diameter and the pipe wall thickness as indicted by generally accepted equations for calculating column buckling. In the Eulerian equation below for buckling of cylindrical cross sections under axial end loading, the critical buckling load Pcr is a function of material properties (properties of steel are most often applicable in the case of coiled tubing) and the outer and inner diameters of the cylindrical column (note: outer diameter minus inner diameter divided by two equals wall thickness).Pcr=4π3E(D4−d4)/64L2                 E=Youngs's Modulus for steel (3×107)        D=outer diameter        d=inner diameter        L=length        
Further, the above equation illustrates that as the length of deployed tubing increases the load at which that tubing will buckle decreases. In a typical extended reach well (thousands of feet deep) readily coiled tubing buckles due to the friction loading between lower portions of the tubing and the walls of the wellbore. Once buckling occurs such frictional loading increases and ultimately exceeds the capacity of the surface equipment and/or the coiled tubing to sustain further loading. At that point, the coiled tubing has gone as far into the wellbore as it can go. Thus, the structural characteristics of coiled tubing limits the capability of using coiled tubing as a conveyance member for logging tools in highly deviated, horizontal, or up-hill sections of wellbores.
Further exacerbating the aforementioned buckling issue is the fact that coiled tubing is supplied from the manufacturer on a reel. For practical transportation and handling matters such reels have size (outer diameter) limits that are not to be exceeded. As such, coiled tubing is plastically deformed when reeled at the manufacturing mill because it must be made to fit on a given reel regardless of its cross-sectional diameter. The tubing is not only deformed axially by such installation on the reel, it is also deformed cross-sectionally such that it assumes a permanent ovality. Such “factory” ovality specifications are published by the various manufacturers of coiled tubing. The capability of employing coiled tubing in highly deviated, horizontal, or up-hill sections is therefore further limited due to the ovality of the coiled tubing because the ovality decreases the buckling resistance of the tubing. The ovality in conjunction with the residual axial strain (from being reeled) also causes the tubing to assume an inherent helical profile when deployed in a wellbore and therefore at even relatively small axial compression loads the tubing winds helically against the wall of the well bore thereby increasing its frictional engagement of that wall. Ovality also decreases the ability of the tube to resist collapse under external differential pressure. Thus, the ovality limits of coiled tubing also limits the capability of using coiled tubing as a conveyance member for logging tools in highly deviated, horizontal, or up-hill sections of wellbores.
Jointed pipe has been used for the deployment of certain downhole devices even where “pushing” is required. In a given diameter range jointed pipe has greater buckling resistance than any of wireline, slickline, or coiled tubing. Each threaded connection (typically every thirty feet) in a string of jointed pipe acts as a column stiffener and upset threaded connections also tend to stand the bulk of the pipe away from the wall of the wellbore thereby reducing cumulative frictional engagement. Jointed pipe is deficient in that it requires a rig (including some form of derrick or crane) for deployment and deployment is very time consuming. Each threaded connection must be made and unmade when correspondingly deploying or retrieving jointed pipe. The additional time consumption and the logistics of moving a rig onto a work location make the use of jointed pipe very expensive as compared with reeled deployment options such as wireline, slickline, and coiled tubing.
A need therefore exists for a reliable and operationally efficient apparatus and method to convey and operate a wellbore tool, such as a logging tool, in a wellbore which is deviated from the vertical.